Winds

The plate contains four maps: Annual Winds, March Winds, August Winds and Annual Maximum Wind Speed. For each of the first three maps the frequency of wind, the mean speed of wind, and the frequency of calms are shown. On all three maps the frequency scale is 1 millimetre equals 2 percent. These values are presented for 50 locations across Canada. The map on the annual maximum wind speed shows the annual maximum hourly wind speeds in kilometres per hour for 30 year return period. Various types of anemometers can be used to measure wind. In Canada, two types are used; both employ a set of rotating cups mounted on a mast to indicate wind speed and a weather vane to indicate direction. There are about 200 anemometers of each type in use at the present time. Most anemometers are mounted according to World Meteorological Organization standards, at a height of 10 metres above the effective terrain. That is to say, the anemometer would be 10 metres above ground level over grassland, and 10 metres above the tree tops in a heavily treed area. Some wind observations were taken in Canada as early as 1840. Hourly observations became very useful with the advent of aircraft, and stations taking hourly observations of wind proliferated in Canada under the Commonwealth Air Training Program during the Second World War. Wind is air in motion. This motion is initiated by the air pressure gradient. The pressure distribution over the earth's surface is controlled by the temperature regime which, generally speaking, is hot near the equator and cold at the poles. The tendency for the wind to blow from high to low pressure is modified by the rotation of the earth. This effect, known as the Coriolis force, causes the wind to deflect to the right in the northern hemisphere. Because of the unequal heating of the earth's surface and the Coriolis force, three main wind zones can be identified in the northern hemisphere. There is a zone of persistent northeast trade winds between the equator and approximately 20oN. From approximately 30oN to 60oN is a zone of mainly westerly winds and from approximately 65°N to the pole is a zone of north-easterly winds. Winds in the latter two zones are not nearly as persistent as the trade winds in either speed or direction.